The invention generally relates to a drum mixer asphalt plant used to produce a variety of asphalt compositions. More directly, the invention relates to a drum mixer in which a first region contains a heating/drying zone and a second region doubles as combustion and mixing zones, to shorten the drum cylinder's overall length, and in which the combustion zone is separated into multiple chambers to stage combustion for greater efficiency, reduced emissions and isolation of hot combustion gases from materials containing hydrocarbons.
Several techniques and numerous equipment arrangements for the preparation of asphaltic cement, also referred by the trade as "hotmix" or "HMA", are known in the prior art. Particularly relevant to the present invention is the production of asphalt compositions in a drum mixer asphalt plant. Typically, water-laden virgin aggregates are heated and dried within a rotating, open-ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. As the virgin aggregate flows through the drum mixer, it is combined with liquid asphalt and mineral binder to produce an asphaltic composition as the desired end-product. The drum mixer also generates, as by-products, a gaseous hydrocarbon emission (known as blue smoke) and sticky dust particles covered with asphalt.
Exposing the liquid asphalt to excessive temperatures within the drum mixer or in close proximity with the burner flame causes serious product degradation, in addition to health and safety hazards. In such event, the more volatile organic compounds (VOCs) of the asphalt are released and the final product may become unfit for use in paving operations. It is desirable to retain the VOCs, within the final product, to render it more flexible and workable. Also, excessive heating of an asphalt composition results in a substantial air pollution control problem, due to the blue-smoke that is produced when hydrocarbon constituents in the asphalt are driven off and released into the atmosphere. Significant investments and efforts have been made by the industry in attempting to control blue-smoke emissions.
Optionally, prior to mixing the virgin aggregate and liquid asphalt, reclaimed asphalt pavement (RAP) may be added once it is ground to a suitable size. The RAP is mixed with the virgin aggregate, in the drum mixer, at a point prior to mixing with the liquid asphalt. The asphalt within the RAP creates the same problems as discussed above in connection with the liquid asphalt. The VOCs within the RAP are released upon exposure to high temperatures and carried in the exhaust gases to the air pollution control equipment, typically a baghouse. Within the baghouse, the blue-smoke condenses on the filter bags and the asphalt-covered dust particles stick to and plug-up the filter bags, thereby presenting a serious fire hazard and reducing their efficiency and useful life.
Conventional systems attempt to avoid the above-noted problems by using a "counter-flow" technique in which the flames and hot gas stream are directed in a direction opposite to the direction of movement of the aggregate material.
One conventional system (U.S. Pat. No. 2,421,345) discloses a counter-flow drum mixer having an aggregate feeder located at an inlet end and a burner head located at a material discharge end opposite to the inlet end. The discharge end of the drum concentrically communicates with, and extends into, a stationary cylindrical casing. The overlapping portions of the drum and casing form a mixing zone therebetween. Mixing blades are affixed to the drum and extend radially outward to the casing. As the drum rotates the blades mix the aggregate with a binder added through a spray bar extending into the mixing zone from the discharge end. To prevent the aggregate/binder mixture from directly contacting a flame from the burner head, while in the mixing zone, an annular shield is axially mounted in the drum to extend through the mixing zone. This shield serves as a conduit for the gases discharged by the burner.
However, as taught by a more recent conventional system (U.S. Pat. No. 4,955,722) the system of the '345 patent was unable to incorporate spent coatings, such as RAP, into the aggregate/binder mixture. Also, in the system of the '345 patent, the burner flame was generated in the mixing zone, thereby giving rise to the formation of bitumen vapors, even when the annular shield is mounted in the center of the mixing chamber.
In recent counter-flow systems (such as U.S. Pat. No. 4,787,938, hereby incorporated by reference), the burner head is extended into, and is located at an intermediate point within, the drum cylinder. These counter-flow mixer drums characteristically include three zones (see U.S. Pat. Nos. 4,892,411; 4,910,540; 4,913,552; 4,948,261; 4,954,995; 4,988,207 and 5,054,931). The three zones include a combustion zone beginning immediately downstream of the burner head, a heating/drying zone further downstream which extends from the combustion zone to the opposite end of the drum (i.e., the gas discharge end) and a mixing zone which extends from the burner head upstream to the outlet end of the drum (i.e., the product discharge end).
When the virgin aggregate is loaded at the gas discharge end in the heating/drying zone, it is cascaded through the drum mixer and shifted upstream past the combustion zone and toward the product discharge end. The RAP, liquid asphalt and fines are added to the aggregate material at varying points behind or upstream the burner head, between the burner head and the outlet end, to avoid direct exposure to the hot gases. To further isolate the RAP and liquid asphalt from the flame, these systems propose surrounding the flame with a burner shield. The aggregate and RAP pass along the outside of the shield, while the flames and gas pass through its center. The system of the '995 patent facilitates isolation by using vanes along the inner perimeter of the mixer drum and adjacent the flame to carry the material beyond the burner head and flames. The system of the '540 patent achieves isolation by enclosing the burner head and flame within first and second telescoping pipes. The telescoping pipes run from the burner head, intermediate the drum, along a majority of the remaining length of the mixer.
However, none of the conventional counter-flow systems are readily incorporated into existing concurrent flow mixer drums (i.e. drums in which the aggregate and hot gas stream are introduced at the same end and travel in the same direction). The above noted counter-flow systems, that are able to combine RAP, liquid asphalt, fines and aggregate, use mixing, combustion, and heating/drying zones arranged end-to-end along the length of the drum mixer, thereby requiring an extremely long and specially designed drum cylinder. Conventional concurrent-flow systems use shorter drum cylinders, and thus cannot be converted to a counter-flow system since the drum cylinders are too short to accommodate the three stage arrangement.
Further, none of the conventional counter-flow systems are readily incorporated into existing counter-flow batch-plant dryers. Briefly, a batch-plant dryer includes a cylindrical mixer drum receiving aggregate at an inlet end and producing a hot gas stream at a discharge end. The aggregate is heated and dried in the mixer drum as it flows in a direction opposite to the hot gas stream and expelled at the discharge end. Once expelled from the dryer, the hot aggregate is carried via a bucket elevator to a batch tower where the aggregate is mixed with liquid asphalt, dumped into a truck and carried to the job site. However, these batch-plant dryers are also to short to accommodate the three-stage arrangement of the previous counter-flow systems.
Moreover, past concurrent-flow mixer drums experience low heating efficiency, thereby limiting the percentage of RAP which may be used within the resulting asphalt composition. Additional inefficiencies result, in both counter-flow and concurrent-flow systems, from veiling of the aggregate material through the flame which quenches the flame.
Further, conventional concurrent-flow mixer drums offer little, if any control, over the temperature of the flame and hot gas stream within the combustion zone, typically heating to a temperature of 3200.degree. F. or more. At such high temperatures, an undesirably large amount of nitrogen oxide (NOX) is produced within the combustion zone. Conventional counter-flow mixer drums attempt to minimize the concentration of NOX emitted by the combustion zone by significantly increasing the volume of air that is blown through the combustion zone. This increase in air flow reduces the NOX emissions in two ways. First, it dilutes the percentage of NOXs in a given volume of air and, second, it reduces the temperature within the combustion zone thereby diminishing the quantity of NOX that is produces.
However, increasing the volume of air flowing through the combustion zone creates other problems. First, it requires a larger blower fan to generate the air and a larger baghouse to filter the exhaust gases emitted by the mixer drum, thereby increasing the systems overall cost. In fact, past counter-flow systems typically operate with an air volume 11/2 to 3 times greater than that necessary for complete combustion of the fuel. Secondly, increasing the air volume may reduce the temperature within the combustion zone below a level necessary for complete combustion of the fuel. When operating below this minimum temperature, the combustion zone produces excess carbon monoxide (CO), which is also undesirable. Consequently, previous counter-flow systems continuously performed a balancing act to minimize NOX emissions without over-cooling the combustion zone and producing CO emissions.
Finally, most conventional counter-flow mixer drums cannot provide adequate radiant heat from the combustion chamber to the mixing zone since the entire mixing zone is upstream o% the combustion zone. Some conventional counter-flow systems allow material, including at least virgin aggregate, to pass through the combustion zone thereby quenching the flame and reducing the overall efficiency.
The need remains in the asphalt industry for improved drum mixer design and operating techniques to address the problems and drawbacks heretofore experienced. The primary objective of this invention is to meet this need.